Variable load refrigeration system particularly for cryogenic temperatures

ABSTRACT

A method for generating refrigeration, especially over a wide temperature range including cryogenic temperatures, wherein a non-toxic, non-flammable and low or non-ozone-depleting mixture is formed from defined components and maintained in variable load form through compression, cooling, expansion and warming steps in a refrigeration cycle.

This is a Division of prior U.S. application(s) Ser. No. 09/222,809Filing Date: Dec. 30, 1998 now U.S. Pat. No. 6,076,372.

TECHNICAL FIELD

This invention relates generally to refrigeration and, moreparticularly, to the use of multiple component refrigerant fluids usefulfor generating refrigeration. The invention is particularly useful forproviding refrigeration down to cryogenic temperatures.

BACKGROUND ART

Refrigeration is conventionally generated by compressing and thenexpanding a refrigerant fluid within a refrigeration circuit. Well knownexamples of such conventional systems include refrigerators and airconditioners. Typically the refrigerant is a single component fluidwhich undergoes a phase change at a required temperature from a liquidto a gas thus making its latent heat of vaporization available forcooling purposes. The efficiency of the conventional system can beimproved by using a multiple component fluid as the refrigerant whichcan provide variable amounts of refrigeration over a requiredtemperature range. However, known multiple component fluid refrigerationcycles cannot effectively provide refrigeration over a large temperaturerange down to colder cryogenic temperatures. Moreover, most well knownrefrigerant fluids are toxic, flammable and/or ozone depleting.

Accordingly it is an object of this invention to provide a method forgenerating refrigeration using a multiple component refrigerant fluidwhich can provide refrigeration over a large temperature range down tocryogenic temperatures.

It is another object of this invention to provide a multiple componentrefrigerant fluid which is non-toxic, non-flammable, and low ornon-ozone-depleting.

SUMMARY OF THE INVENTION

The above and other objects, which will become apparent to those skilledin the art upon a reading of this disclosure, are attained by thepresent invention, one aspect of which is:

A method for generating refrigeration comprising:

(A) compressing a variable load refrigerant mixture comprising at leastone component from the group consisting of fluorocarbons,hydrofluorocarbons and fluoroethers and at least one component from thegroup consisting of fluorocarbons, hydrofluorocarbons,hydrochlorofluorocarbons, fluoroethers, atmospheric gases andhydrocarbons to produce a compressed variable load refrigerant mixture;

(B) cooling the compressed variable load refrigerant mixture to producea cooled compressed variable load refrigerant mixture;

(C) expanding the cooled, compressed variable load refrigerant mixtureand generating refrigeration to produce a lower temperature variableload refrigerant mixture; and

(D) warming the lower temperature variable load refrigerant mixture.

Another aspect of the invention is:

A refrigerant mixture which is non-toxic, non-flammable andlow-ozone-depleting comprising at least one component from the groupconsisting of fluorocarbons, hydrofluorocarbons and fluoroethers and atleast one component from the group consisting of fluorocarbons,hydrofluorocarbons, hydrochlorofluorocarbons, fluoroethers, atmosphericgases and hydrocarbons.

As used herein the term “variable load refrigerant” means a mixture oftwo or more components in proportions such that the liquid phase ofthose components undergoes a continuous and increasing temperaturechange between the bubble point and the dew point of the mixture. Thebubble point of the mixture is the temperature, at a given pressure,wherein the mixture is all in the liquid phase but addition of heat willinitiate formation of a vapor phase in equilibrium with the liquidphase. The dew point of the mixture is the temperature, at a givenpressure, wherein the mixture is all in the vapor phase but extractionof heat will initiate formation of a liquid phase in equilibrium withthe vapor phase. Hence, the temperature region between the bubble pointand the dew point of the mixture is the region wherein both liquid andvapor phases coexist in equilibrium. In the practice of this inventionthe temperature differences between the bubble point and the dew pointfor the variable load refrigerant is at least 10° K, preferably at least20° K and most preferably at least 50° K.

As used herein the term “fluorocarbon” means one of the following:tetrafluoromethane (CF₄), perfluoroethane (C₂F₆), perfluoropropane(C₃F₈), perfluorobutane (C₄F₁₀), perfluoropentane (C₅F₁₂),perfluoroethene (C₂F₄), perfluoropropene (C₃F₆), perfluorobutene (C₄F₈),perfluoropentene (C₅F₁₀), hexafluorocyclopropane (cyclo-C₃F₆) andoctafluorocyclobutane (cyclo-C₄F₈).

As used herein the term “hydrofluorocarbon” means one of the following:fluoroform (CHF₃), pentafluoroethane (C₂HF₅), tetrafluoroethane(C₂H₂F₄), heptafluoropropane (C₃HF₇), hexafluoropropane (C₃H₂F₆),pentafluoropropane (C₃H₃F₅), tetrafluoropropane (C₃H₄F₄),nonafluorobutane (C₄HF₉), octafluorobutane (C₄H₂F₈), undecafluoropentane(C₅HF₁₁), methyl fluoride (CH₃F), difluoromethane (CH₂F₂), ethylfluoride (C₂H₅F), difluoroethane (C₂H₄F₂), trifluoroethane (C₂H₃F₃),difluoroethene (C₂H₂F₂), trifluoroethene (C₂HF₃), fluoroethene (C₂H₃F),pentafluoropropene (C₃HF₅), tetrafluoropropene (C₃H₂F₄),trifluoropropene (C₃H₃F₃), difluoropropene (C₃H₄F₂), heptafluorobutene(C₄HF₇), hexafluorobutene (C₄H₂F₆) and nonafluoropentene (C₅HF₉).

As used herein the term “hydrochlorofluorocarbon” means one of thefollowing: chlorodifluoromethane (CHClF₂), chlorofluoromethane (CH₂ClF),chloromethane (CH₃Cl), dichlorofluoromethane (CHCl₂F),chlorotetrafluoroethane (C₂HClF₄), chlorotrifluoroethane (C₂H₂ClF₃),chlorodifluoroethane (C₂H₃ClF₂), chlorofluoroethane (C₂H₄ClF),chloroethane (C₂H₅Cl), dichlorotrifluoroethane (C₂HCl₂F₃),dichlorodifluoroethane (C₂H₂Cl₂F₂), dichlorofluoroethane (C₂H₃Cl₂F),dichloroethane (C₂H₄Cl₂), trichlorofluoroethane (C₂H₂Cl₃F),trichlorodifluoroethane (C₂HCl₃F₂), trichloroethane (C₂H₃Cl₃),tetrachlorofluoroethane (C₂HCl₄F), chloroethene (C₂H₃Cl), dichloroethene(C₂H₂Cl₂), dichlorofluoroethene (C₂H₂ClF) and dichlorodifluoroethene(C₂HClF₂).

As used herein the term “fluoroether” means one of the following:trifluoromethyoxy-perfluoromethane (CF₃—O—CF₃),difluoromethoxy-perfluoromethane (CHF₂—O—CF₃),fluoromethoxy-perfluoromethane (CH₂F—O—CF₃),difluoromethoxy-difluoromethane (CHF₂—O—CHF₂),difluoromethoxy-perfluoroethane (CHF₂—O—C₂F₅),difluoromethoxy-1,2,2,2-tetrafluoroethane (CHF₂—O—C₂HF₄),difluoromethoxy-1,1,2,2-tetrafluoroethane (CHF₂—O—C₂HF₄),perfluoroethoxyfluoromethane (C₂F₅—O—CH₂F),perfluoromethoxy-1,1,2-trifluoroethane (CF₃—O—C₂H₂F₃),perfluoromethoxy-1,2,2-trifluoroethane (CF₃O—C₂H₂F₃),cyclo-1,1,2,2-tetrafluoropropylether (Cyclo-C₃H₂F₄—O—),cyclo-1,1,3,3-tetrafluoropropylether (cyclo-C₃H₂F₄—O—),perfluoromethoxy-1,1,2,2-tetrafluoroethane (CF₃—O—C₂HF₄),cyclo-1,1,2,3,3-pentafluoropropylether (cyclo-C₃H₅—O—),perfluoromethoxy-perfluoroacetone (CF₃—O—CF₂—O—CF₃),perfluoromethoxy-perfluoroethane (CF₃—O—C₂F₅),perfluoromethoxy-1,2,2,2-tetrafluoroethane (CF₃—O—C₂HF₄),perfluoromethoxy-2,2,2-trifluoroethane (CF₃—O—C₂H₂F₃),cyclo-perfluoromethoxy-perfluoroacetone (cyclo-CF₂—O—CF₂—O—CF₂—) andcyclo-perfluoropropylether (cyclo-C₃F₆—O).

As used herein the term “atmospheric gas” means one of the following:nitrogen (N₂), argon (Ar), krypton (Kr), xenon (Xe), neon (Ne), carbondioxide (CO₂), oxygen (O₂) and helium (He).

As used herein the term “hydrocarbon” means one of the following:hydrogen (H₂), methane (CH₄), ethane (C₂H₆), ethene (C₂H₄), propane(C₃H₈), propene (C₃H₆), butane (C₄H₁₀), butene (C₄H₈), cyclopropane(C₃H₆) and cyclobutane (C₄H₈).

As used herein the term “non-toxic” means not posing an acute or chronichazard when handled in accordance with acceptable exposure limits.

As used herein the term “non-flammable” means either having no flashpoint or a very high flash point of at least 600° K.

As used herein the term “low-ozone-depleting” means having an ozonedepleting potential less than 0.15 as defined by the Montreal Protocolconvention wherein dichlorofluoromethane (CCl₂F₂) has an ozone depletingpotential of 1.0.

As used herein the term “non-ozone-depleting” means having no componentwhich contains a chlorine, bromine or iodine atom.

As used herein the term “normal boiling point” means the boilingtemperature at 1 standard atmosphere pressure, i.e. 14.696 pounds persquare inch absolute.

As used herein the term “cryogenic temperature” means a temperature of150° K or less.

As used herein the term “indirect heat exchange” means the bringing oftwo fluids into heat exchange relation without any physical contact orintermixing of the fluids with each other.

As used herein the term “expansion” means to effect a reduction inpressure.

As used herein the terms “turboexpansion” and “turboexpander” meanrespectively method and apparatus for the flow of high pressure fluidthrough a turbine to reduce the pressure and the temperature of thefluid thereby generating refrigeration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a generalized temperature versus concentration diagram for avariable load refrigerant mixture at a given pressure.

FIG. 2 is a schematic representation of a system wherein the inventionmay be practiced.

FIG. 3 is a schematic representation of another system wherein theinvention may be practiced.

FIG. 4 is a schematic representation of a three loop system wherein theinvention may be used to provide refrigeration over a wide temperaturerange.

DETAILED DESCRIPTION

The invention comprises a refrigerant mixture composed of definedcomponents in proportions which form a variable load refrigerant mixtureand the use of such refrigerant mixture in a refrigeration cycle. Thevariable load refrigerant mixture can be in all gas, gas/liquid, or allliquid phases depending on the process and the position within theprocess, i.e. the heat exchange position (top, middle, bottom).Preferably the cycle is a closed loop cycle. The variable loadrefrigerant mixtures show a smooth temperature change accompanying aphase change. This is demonstrated in FIG. 1, a temperature versusconcentration diagram of a variable load refrigerant mixture at a givenpressure. With any given mix of components A and B (xmix) at temperature(tmix), two phases will exist, the composition of the saturated vapor(xmixv) will differ from the liquid in equilibrium with the vapor andthe liquid will have the composition (xmixl). As the temperature islowered, both the liquid phase composition and the vapor phasecomposition will change, each becoming enriched in component B. Thecondensing mixture is constantly changing its composition and thus itscondensing temperature. It is this feature that makes it possible toimprove the performance of a refrigeration cycle. The cycle improvementis related to the use of multiple components, each with its own normalboiling point and associated latent heat of vaporization. The properselection of the refrigerant components, optimum concentrations in themixture, along with operating pressure levels, and refrigerant cycles,allows the generation of variable amounts of refrigeration over therequired temperature range. The provision of the variable refrigerationas a function of the temperature allows the optimum control of heatexchange temperature differences within the refrigeration user systemand thereby reduces system energy requirements.

The variable load refrigerant mixture of this invention comprises atleast one component from the group consisting of fluorocarbons,hydrofluorocarbons and fluoroethers and at least one component from thegroup consisting of fluorocarbons, hydrofluorocarbons,hydrochlorofluorocarbons, fluoroethers, atmospheric gases andhydrocarbons.

One preferred variable load refrigerant mixture of this inventioncomprises at least two components from the group consisting offluorocarbons, hydrofluorocarbons and fluoroethers and at least onecomponent from the group consisting of fluorocarbons,hydrofluorocarbons, hydrochlorofluorocarbons, fluoroethers, atmosphericgases and hydrocarbons.

Another preferred variable load refrigerant mixture of this inventioncomprises at least one fluorocarbon and at least one component from thegroup consisting of hydrofluorocarbons and atmospheric gases.

Another preferred variable load refrigerant mixture of this inventioncomprises at least one fluorocarbon, at least one hydrofluorocarbon andat least one atmospheric gas.

Another preferred variable load refrigerant mixture of this inventioncomprises at least three components from the group consisting offluorocarbons, hydrofluorocarbons and fluoroethers and at least onecomponent from the group consisting of fluorocarbons,hydrofluorocarbons, hydrochlorofluorocarbons, fluoroethers, hydrocarbonsand atmospheric gases.

Another preferred variable load refrigerant mixture of this inventioncomprises at least two components from the group consisting offluorocarbons, hydrofluorocarbons and fluoroethers and at least oneatmospheric gas.

Another preferred variable load refrigerant mixture of this inventioncomprises at least two components from the group consisting offluorocarbons, hydrofluorocarbons and fluoroethers, at least oneatmospheric gas and at least one component from the group consisting offluorocarbons, hydrofluorocarbons, hydrochlorofluorocarbons,fluoroethers, hydrocarbons and atmospheric gases.

Another preferred variable load refrigerant mixture of this inventioncomprises at least two components from the group consisting offluorocarbons, hydrofluorocarbons and fluoroethers and at least twodifferent atmospheric gases.

Another preferred variable load refrigerant mixture of this inventionincludes at least one fluoroether, i.e. comprises at least onefluoroether and at least one component from the group consisting offluorocarbons, hydrofluorocarbons, fluoroethers,hydrochlorofluorocarbons, hydrocarbons and atmospheric gases.

In one preferred embodiment of the invention the variable loadrefrigerant mixture contains no hydrochlorofluorocarbons. In anotherpreferred embodiment of the invention the variable load refrigerantcontains no hydrocarbons. Most preferably the variable load refrigerantcontains neither hydrochlorofluorocarbons nor hydrocarbons. Mostpreferably the variable load refrigerant is non-toxic, non-flammable andnon-ozone-depleting and every component of the variable load refrigerantmixture is either a fluorocarbon, hydrofluorocarbon, fluoroether oratmospheric gas.

In one preferred embodiment of the invention the variable loadrefrigerant consists solely of fluorocarbons. In another preferredembodiment of the invention the variable load refrigerant consistssolely of fluorocarbons and hydrofluorocarbons. In another preferredembodiment of the invention the variable load refrigerant consistssolely of fluorocarbons and atmospheric gases. In another preferredembodiment of the invention the variable load refrigerant consistssolely of fluorocarbons, hydrofluorocarbons and fluoroethers. In anotherpreferred embodiment of the invention the variable load refrigerantconsists solely of fluorocarbons, fluoroethers and atmospheric gases.Most preferably every component of the variable load refrigerant iseither a fluorocarbon, hydrofluorocarbon, fluoroether or atmosphere gas.

The invention is particularly advantageous for use in efficientlyreaching cryogenic temperatures from ambient temperatures. Tables 1-15list preferred examples of variable load refrigerant mixtures of thisinvention. The concentration ranges given in the Tables are in the molepercent. The examples shown in Tables 1-5 are preferred mixtures forgenerating refrigeration above about 200° K and the examples shown inTables 6-15 are preferred mixtures for generating refrigeration belowabout 200° K.

TABLE 1 COMPONENT CONCENTRATION RANGE C₅F₁₂ 5-35 C₄F₁₀ 0-25 C₃F₈ 10-50 C₂F₆ 10-60  CF₄ 0-25

TABLE 2 COMPONENT CONCENTRATION RANGE C₅F₁₂ 5-35 C₃H₃F₆ 0-25 C₃F₈ 10-50 CHF₃ 10-60  CF₄ 0-25

TABLE 3 COMPONENT CONCENTRATION RANGE C₃H₃F₅ 5-35 C₃H₃F₆ 0-25 C₂H₂F₄5-20 C₂HF₅ 5-20 C₂F₆ 10-60  CF₄ 0-25

TABLE 4 COMPONENT CONCENTRATION RANGE CHF₂—O—C₂HF₄ 5-35 C₄F₁₀ 0-25CF₃—O—CHF₂ 10-25  CF₃—O—CF₃ 0-20 C₂F₆ 10-60  CF₄ 0-25

TABLE 5 COMPONENT CONCENTRATION RANGE CHF₂—O—C₂HF₄ 5-35 C₃H₂F₆ 0-25CF₃-O-CHF₂ 10-50  CHF₃ 10-60  CF₄ 0-25

TABLE 6 COMPONENT CONCENTRATION RANGE C₅F₁₂ 5-25 C₄F₁₀ 0-15 C₃F₈ 10-40 C₂F₆ 0-30 CF₄ 10-50  Ar 0-40 N₂ 10-80 

TABLE 7 COMPONENT CONCENTRATION RANGE C₃H₃F₅ 5-25 C₄F₁₀ 0-15 C₃F₈ 10-40 CHF₃ 0-30 CF₄ 10-50  Ar 0-40 N₂ 10-80 

TABLE 8 COMPONENT CONCENTRATION RANGE C₃H₃F₅ 5-25 C₃H₃F₆ 0-15 C₂H₂F₄0-20 C₂HF₅ 5-20 C₂F₆ 0-30 CF₄ 10-50  Ar 0-40 N₂ 10-80 

TABLE 9 COMPONENT CONCENTRATION RANGE CHF₂—O—C₂HF₄ 5-25 C₄H₁₀ 0-15CF₃—O—CHF₂ 10-40  CF₃—O—CF₃ 0-20 C₂F₆ 0-30 CF₄ 10-50  Ar 0-40 N₂ 10-80 

TABLE 10 COMPONENT CONCENTRATION RANGE C₃H₃F₅ 5-25 C₃H₂F₆ 0-15CF₃—O—CHF₂ 10-40  CHF₃ 0-30 CF₄ 0-25 Ar 0-40 N₂ 10-80 

TABLE 11 COMPONENT CONCENTRATION RANGE C₅F₁₂ 5-25 C₄F₁₀ 0-15 C₃F₈ 10-40 C₂F₆ 0-30 CF₄ 10-50  Ar 0-40 N₂ 10-80  Ne 0-10 He 0-10

TABLE 12 COMPONENT CONCENTRATION RANGE C₃H₃F₅ 5-25 C₄F₁₀ 0-15 C₃F₈10-40  CHF₃ 0-30 CF₄ 10-50  Ar 0-40 N₂ 10-80  Ne 0-10 He 0-10

TABLE 13 COMPONENT CONCENTRATION RANGE C₃H₃F₅ 5-25 C₃H₂F₆ 0-15 C₂H₂F₄5-20 C₂HF₅ 5-20 C₂F₆ 0-30 CF₄ 10-50  Ar 0-40 N₂ 10-80  Ne 0-10 He 0-10

TABLE 14 COMPONENT CONCENTRATION RANGE CHF₂—O—C₂HF₄ 5-25 C₄F₁₀ 0-15CF₃—O—CHF₂ 10-40  CF₃—O—CF₃ 0-20 C₂F₆ 0-30 CF₄ 10-50  Ar 0-40 N₂ 10-80 Ne 0-10 He 0-10

TABLE 15 COMPONENT CONCENTRATION RANGE C₃H₃F₅ 5-25 C₃H₂F₆ 0-15CF₃—O—CHF₃ 10-40  CHF₃ 0-30 CF₄ 0-25 Ar 0-40 N₂ 10-80  Ne 0-10 He 0-10

FIG. 2 illustrates one refrigeration cycle wherein the invention may bepracticed. Referring now to FIG. 2, the variable load refrigerantmixture of this invention recirculates in a refrigeration circuit orloop 1. Refrigerant 2 is compressed by passage through compressor 3 toform compressed refrigerant fluid 4, cooled to near ambient temperatureby passage through aftercooler 70, and then cooled and preferably atleast partially liquefied by passage through heat exchanger 5. Unlessotherwise specified, each heat exchange step illustrated in the Drawingsis an indirect heat exchange step. Cooled refrigerant fluid 6 is thenthrottled, i.e. expanded, through valve 7 to a lower pressure. Thepressure expansion can be accomplished by a turbine, such as a gasexpansion, two-phase expansion, or liquid expansion turbine. Therefrigeration produced can be utilized at a single or narrow temperaturelevel by cooling a fluid 8 by indirect heat exchange in heat exchanger9, or can be utilized over a much wider temperature range in heatexchanger 5. The refrigeration may be used to cool one or more fluidstreams passing through heat exchanger 5 as illustrated bycountercurrent stream 10 and cocurrent stream 11. Although on an overallbasis, stream 11 is shown as being heated in exchanger 5, on a localbasis it can be cooled within exchanger 5. The resulting warmedrefrigerant mixture is then passed as stream 2 to compressor 3 and thecycle repeats.

The cooling arrangement could also include a precooler circuit or loop12 wherein a variable load refrigerant mixture 13 of this inventiondesigned to provide refrigeration at intermediate temperature levels iscompressed in precooler compressor 14, cooled to ambient temperature inaftercooler 71, and the resulting compressed fluid 15 is cooled in heatexchanger 5. The resulting cooled fluid 16 is throttled through a valveor a suitable turbine 17 to generate refrigeration and resulting lowertemperature refrigerant fluid 18 is warmed and then cycled as stream 13to compressor 14.

The effect of the precooler loop can be accomplished by intermediateremoval of some of the refrigerant mixture and recycling of liquid asillustrated in FIG. 3. The liquid recycle feature provides processflexibility in matching the refrigerant mixtures to the requiredtemperature ranges and avoids unnecessary cooling and potential freezingof the liquid refrigerant. The numerals in FIG. 3 are the same as thosein FIG. 2 for the common elements which will not be described again indetail. Referring now to FIG. 3, refrigerant fluid 20 is compressed bypassage through compressor 21 to form compressed refrigerant fluid 22which is cooled of the heat of compression to near ambient temperatureby aftercooler 71 and then cooled and partially condensed by partialtraverse of heat exchanger 5. Cooled two phase refrigerant mixture 23 ispassed into phase separator 24 wherein it is separated into vapor andliquid. Vapor 25 is further cooled through heat. exchanger 5, throttledthrough valve 26 and warmed by passage through heat exchanger 9 and/or5. Liquid 27 is passed through valve 28 and then vaporized by passagethrough heat exchanger 5. In the embodiment illustrated in FIG. 3 theliquid is combined with the lower pressure vapor which is throttledthrough valve 26 prior to vaporization. The resulting warmed refrigerantmixture is then returned as stream 29 to compressor 21 and therefrigeration cycle begins anew. Although a single phase separation isillustrated, it is understood that multiple phase separations atdifferent temperature levels could be utilized to provide stagedprecooling circuits.

The invention is particularly useful for providing refrigeration fromambient temperature down to cryogenic temperature, even down to as low atemperature as 5° K. While the invention may be used to provide suchrefrigeration over this entire temperature range in a single loop, it isgenerally preferable to provide this refrigeration in a plurality ofcascade loops. The use of multiple cascade loops allows each circuit toprovide refrigeration over a selected temperature range. Thereby theselection of a suitable refrigerant mixture is facilitated, since theselected mixture need only be operable over a more limited temperaturerange. Note that although each cascade circuit is intended to providerefrigeration primarily over its associated temperature range, it mayalso provide some refrigeration at higher temperature levels. Thus thecascade circuits may somewhat overlap each other with respect toproviding refrigeration at a given temperature range.

The cascade loop system is illustrated in and discussed in conjunctionwith FIG. 4. Referring now to FIG. 4 higher temperature variable loadrefrigerant fluid comprising two or more of, for example,tetrafluoromethane, fluoroform, perfluoropropane, perfluorobutane,pentafluoropropane, tetrafluoroethane, difluoromethoxy-difluoromethaneand perfluoropentane, recirculates in higher temperature loop 30 whereinrefrigeration is provided from the ambient temperature of about 300° Kdown to about 200° K. The higher temperature refrigerant fluid 31 atabout 300° K is compressed in compressor 32, cooled through cooler 33and heat exchanger 60 and throttled through valve 34 to produce lowertemperature refrigerant fluid 35 at about 200° K. The lower temperaturerefrigerant fluid is then warmed back to about 300° K and returned asstream 31 to compressor 32.

Intermediate temperature variable load refrigerant fluid, which maycontain nitrogen and/or argon in addition to one or more of thecomponents recited for the higher temperature fluid, recirculates inintermediate temperature loop 40 wherein refrigeration is provided fromabout 200° K down to about 100° K. The intermediate temperaturerefrigerant fluid 41 is compressed in compressor 42, cooled throughcooler 43 and heat exchangers 60 and 61, and throttled through valve 44to produce lower temperature refrigerant fluid 45 at about 100° K whichis warmed and then returned as stream 41 to compressor 42.

Very low temperature refrigerant fluid comprising two or more ofnitrogen, argon, helium, neon and hydrogen recirculates in very lowtemperature loop 50 wherein the temperature level is brought from about100° K to about 20° K or even lower. The very low temperaturerefrigerant fluid 51 is compressed in compressor 52, cooled throughcooler 53 and heat exchangers 60, 61 and 62, and throttled through valve54 to produce lower temperature refrigerant fluid 55 at about 20° K orlower which is warmed by passage through warmer 56 and heat exchangers62, 61 and 60 and then returned as stream 51 to compressor 52.

The invention is especially useful for providing refrigeration over awide temperature range, particularly one which encompasses cryogenictemperatures. In a preferred embodiment of the invention each of the twoor more components of variable load refrigerant mixture has a normalboiling point which differs by at least 20 degrees Kelvin from thenormal boiling point of every other component in that refrigerantmixture. This enhances the effectiveness of providing refrigeration overa wide temperature range, particularly one which encompasses cryogenictemperatures. In a particularly preferred embodiment of the invention,the normal boiling point of the highest boiling component of themulticomponent refrigerant fluid is at least 50° K, preferably at least100° K, most preferably at least 200° K, greater than the normal boilingpoint of the lowest boiling component of the multicomponent refrigerantfluid.

The components and their concentrations which make up the refrigerantmixture of this invention are such as to form a variable loadrefrigerant mixture and preferably maintain such a variable loadcharacteristic throughout the whole temperature range of the method ofthe invention. This markedly enhances the efficiency with which therefrigeration can be generated and utilized over such a wide temperaturerange. The defined group of components has an added benefit in that theycan be used to form mixtures which are non-toxic, non-flammable and lowor non-ozone-depleting. This provides additional advantages overconventional refrigerants which typically are toxic, flammable and/orozone-depleting.

One preferred variable load refrigerant mixture of this invention whichis non-toxic, non-flammable and non-ozone-depleting comprises two ormore components from the group consisting of C₅F₁₂, CHF₂—O—C₂HF₄, C₄HF₉,C₃H₃F₅, C₂F₅—O—CH₂F, C₃H₂F₆, CHF₂—O—CHF₂, C₄F₁₀, CF₃—O—C₂H₂F₃, C₃HF₇,CH₂F—O—CF₃, C₂H₂F₄, CHF₂—O—CF₃, C₃F₈, C₂HF₅, C₂F₆, CHF₃, CF₄, O₂, Ar,N₂, Ne and He.

The invention may be used to generate refrigeration for a large numberof uses, especially for cryogenic applications. Among such uses one canname gas separation processes such as cryogenic air separations andother cryogenic separations and natural gas upgrading, liquefiers, foodfreezing, vent gas recovery, heat pumping, cryogenic liquid storage andtransport vessel recondensation, crystallization, solidification, lowtemperature grinding, chemicals storage and transport, biological andmedical material storage and transport, and refrigerated rooms, i.e.cold rooms utilized for materials handling and storage.

Although the invention has been described in detail with reference tocertain preferred embodiments, those skilled in the art will recognizethat there are other embodiments of the invention within the spirit andthe scope of the claims.

What is claimed is:
 1. A refrigerant mixture which is non-toxic,non-flammable and low-ozone-depleting consisting solely of fluorocarbonswherein the components and their concentrations which make up therefrigerant mixture are such as to form a variable load refrigerantmixture, wherein each of the components of the mixture has a normalboiling point which differs by at least 20 degrees Kelvin from thenormal boiling point of each of the other components of the refrigerantmixture, and wherein the normal boiling point of the highest boilingcomponent of the refrigerant mixture is at least 50° K greater than thenormal boiling point of the lowest boiling component of the refrigerantmixture.
 2. A refrigerant mixture which is non-toxic, non-flammable andlow-ozone-depleting consisting solely of fluorocarbons andhydrofluorocarbons wherein the components and their concentrations whichmake up the refrigerant mixture are such as to form a variable loadrefrigerant mixture, wherein each of the components of the mixture has anormal boiling point which differs by at least 20 degrees Kelvin fromthe normal boiling point of each of the other components of therefrigerant mixture, and wherein the normal boiling point of the highestboiling component of the refrigerant mixture is at least 50° K greaterthan the normal boiling point of the lowest boiling component of therefrigerant mixture.
 3. A refrigerant mixture which is non-toxic,non-flammable and low-ozone-depleting consisting solely offluorocarbons, hydrofluorocarbons and fluoroethers wherein thecomponents and their concentrations which make up the refrigerantmixture are such as to form a variable load refrigerant mixture, whereineach of the components of the mixture has a normal boiling point whichdiffers by at least 20 degrees Kelvin from the normal boiling point ofeach of the other components of the refrigerant mixture, and wherein thenormal boiling point of the highest boiling component of the refrigerantmixture is at least 50° K greater than the normal boiling point of thelowest boiling component of the refrigerant mixture.
 4. A refrigerantmixture which is non-toxic, non-flammable and low-ozone-depletingconsisting solely of fluorocarbons, fluoroethers and atmospheric gaseswherein the components and their concentrations which make up therefrigerant mixture are such as to form a variable load refrigerantmixture, wherein each of the components of the mixture has a normalboiling point which differs by at least 20 degrees Kelvin from thenormal boiling point of each of the other components of the refrigerantmixture, and wherein the normal boiling point of the highest boilingcomponent of the refrigerant mixture is at least 50° K greater than thenormal boiling point of the lowest boiling component of the refrigerantmixture.